All drivers must be able to read a number plate from at least 20 metres (65ft) away at all times.

And while new drivers are required to pass an eyesight test before they get their licence, the DVLA recommends road users check their eyesight using the number plate test to allow for deterioration over time.

If motorists become unable to read a number plate - even partially - from the minimum distance, they are legally required to notify the DVLA.

This could then see their licence revoked or refused by the Government agency if safe eyesight standards aren't met.

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Drivers need to be able to read a number plate from at least 20metres awayCredit: Alamy

What are the minimum eyesight standards for drivers?

The DVLA outlines the following legal minimum eyesight requirements for all motorists:

In good daylight, able to read the registration mark fixed to a vehicle registered under current standards

at a distance of 20 metres with letters and numbers 79 mm high by 50 mm wide on a car registered since 1 September 2001 or

at a distance of 20.5 metres with letters and numbers 79 mm high by 57 mm wide on a car registered before 1 September 2001 and

the visual acuity must be at least Snellen 6/12 with both eyes open or in the only eye if monocular

Any driver unable to meet these standards must not drive and must notify the DVLA, which will refuse or revoke a licence.

The law also requires all drivers to have a minimum 120 degree field of vision.

With 48million motorists registered on UK roads, the survey suggests millions could be driving with potentially dangerous eyesight.

The DVLA recommends anyone who thinks their eyesight is deteriorating or finds the number plate test difficult visit an optician before driving again.

Dr Wyn Parry, DVLA’s Senior Doctor, said: "The number plate test is a simple and effective way for people to check their eyesight meets the required standards for driving.

"The easiest and quickest way to do this is to work out what 20 metres looks like at the roadside - this is typically about the length of 5 cars parked next to each other - and then test yourself on whether you can clearly read the number plate.

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"It’s an easy check to perform any time of day at the roadside and takes just a couple of seconds.

"Having good eyesight is essential for safe driving, so it’s really important for drivers to have regular eye tests.

"Eyesight can naturally deteriorate over time so anyone concerned about their eyesight should visit their optician - don’t wait for your next check-up."

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